This invention relates to improved pattern materials, to improved disposable patterns especially made for use in investment casting processes, sometimes also known as lost wax processes, and to an improved investment casting process.
Investment casting processes have been used for centuries. Materials for making disposable patterns to be used in such processes are formulated for a number of properties, including important properties such as dimensional reproduceability and highly accurate surface finish in the molded disposable pattern. Because such properties are critically important to many products made by lost wax processes, continuing efforts are always underway to improve those properties of pattern materials, among others.
Virtually all of the properties of an investment casting depend upon the quality of the disposable pattern. These in turn depend upon the characteristics of the pattern forming materials from which disposable patterns are molded.
Disposable thermoplastic patterns are usually formed by heating and melting a thermoplastic pattern forming composition, introducing the molten composition into a mold, and then cooling the composition until it solidifies to form a disposable pattern. Thereafter, the disposable thermoplastic pattern is removed from the mold, is assembled if necessary with other patterns, and is then encased in a mold forming material, usually a ceramic material, in accordance with one of a variety of known methods, thereby to form a shell or cast about the disposable pattern. The disposable pattern is then removed, as by melting or vaporizing the pattern material, so that it leaves the shell or cast. Thereafter the shell or mold is ready for one-time use for forming an investment cast part. A text describing known procedures used in lost wax processes is entitled "Investment Casting," H. T. Bidwell, Machinery Publishing Co., Ltd., England, 1969.
It is apparent that the surface characteristics of the disposable pattern and of the ceramic shell are transferred, so to speak, to the final casting. It is also apparent that the pattern material and any residue therefrom will affect the surface characteristics and metallurgical characteristics of a casting. Similarly, it is clear that variations in expansion and contraction of compositions from which disposable patterns are formed will result in shells or casts of varying dimensions, which will then produce inconsistent castings. It is for such reasons that the properties of pattern materials are critical to the investment caster.
Many thermoplastic pattern materials have been used and have been suggested for use in the past. As the name "lost wax" process implies, true waxes, such as natural waxes, as beeswax and the like, were originally used as thermoplastic pattern materials. As other pattern materials were sought to improve the properties of disposable patterns, other natural thermoplastic materials, such as gum damar, gum rosin, esparto waxes, and the like, mineral waxes, such as those extracted from soft coal, and the like, and petroleum waxes were adopted for use. Subsequently, modified waxes, such as microcrystalline waxes, were developed for use and used in lost wax processes. More recently, synthetic thermoplastic materials, such as polystyrene, have been used as pattern materials, or as thermoplastic pattern forming composition modifiers as a result of the continuing efforts of researchers to improve upon and develop new thermoplastic materials. Those efforts have also resulted in the use by some investment casters of materials other than thermoplastic pattern materials, such as mixtures of metallic salts, mercury, among others.
Efforts have also been made to increase the dimensional accuracy and stability of thermoplastic pattern forming compositions by the addition of solid filler materials. Polystyrene powder and urea powder have been so used, and have been added in minor quantities to thermoplastic pattern-forming compositions. Organic acids, such as fumaric acid, adipic acid and isophthalic acid, have also sometimes been used as solid filler, usually in amounts of up to about 40% by volume of the thermoplastic pattern forming composition, and in a particle size generally in the range of about 175 to 250 mesh.
In U.S. Pat. No. 3,754,943, issued to Paul Solomon on Aug. 28, 1973, there is disclosed an investment casting composition and method in which cyanuric acid is used as a solid filler in a thermoplastic pattern forming composition. This invention provides improved dimensional stability to the thermoplastic patterns. However, since the filler material is acidic in nature, particularly in the presence of condensed moisture, it tends to be corrosive at high temperatures and damaging to the mold.
U.S. patent application Ser. No. 297,352, filed by Paul Solomon on Oct. 13, 1972, now U.S. Pat. No. 3,887,382 discloses an investment casting composition and method in which decachlorobiphenyl is used as a solid filler in a thermoplastic pattern forming composition. This invention also provides improved dimensional stability to the thermoplastic patterns. However, the decachlorobiphenyl has a very high melting point, above 300.degree. C., and tends to leave a residue in the mold which frequently requires burning off.